Recently, electronic devices are becoming more portable and codeless rapidly. Along with this trend, high-voltage and high-energy density nonaqueous electrolyte secondary batteries are increasingly commercialized as a power supply for driving electronic devices.
The positive electrode for such a nonaqueous electrolyte secondary battery generally contains a lithium composite oxide higher in oxidation-reduction potential such as lithium cobalt oxide, lithium nickel oxide, or lithium manganese oxide. Alternatively, the negative electrode for the nonaqueous electrolyte secondary battery generally contains a carbon material. In addition, a nonaqueous electrolyte containing a lithium salt such as LiClO4 or LiPF6 dissolved in a solvent has been used as the electrolyte for the nonaqueous electrolyte secondary battery. There is a separator placed between the positive electrode and the negative electrode. For example, a microporous film of a polyolefin-based material has been used as the separator.
If short circuiting occurs for some reason in the region of a nonaqueous electrolyte secondary battery where the resistance is relatively lower, there may be large electric current flowing intensely at the point of short circuiting point. In such a case, the battery may be heated to high temperature by rapid heat generation. Various measures are taken in producing a battery, for prevention of the battery from rising to high temperature by such short circuiting.
Specifically, measures to prevent contamination of the battery, for example with metal powders derived from raw materials, dust in the production atmosphere and others, are taken in the production process.
As for the configuration of battery, taken is a measure to prevent internal short-circuiting, by protecting exposed regions of the core material (current collector), which are the regions lower in resistance in the electrode, with an insulation tape. In addition, also used is a so-called shutdown function of shutting down ion current by collapse of micropores at high temperature, by using, for example, a microporous polyethylene film containing micropores that clogs by fusion at a temperature of approximately 135° C. as the separator. Such a separator, if used, stops flow of the short-circuit current and prevents heat generation by collapse of the micropores in separator, even if there is short circuiting generated in battery.
Patent Document 1 proposes a nonaqueous electrolyte secondary battery containing low-conductivity cathode material powder that controls the flow of current and reduces the Joule heat generation in the short circuited region in battery when short circuiting occurs.
Known as a test for determining the reliability during internal short-circuit is an internal short-circuit test by using a nail to be inserted into battery (hereinafter, referred to briefly as nail penetration test). High-energy density lithium secondary batteries releases large energy and are heated to high temperature rapidly when short-circuited in the nail penetration test.
When an lithium ion battery having a separator of a microporous polyethylene film having such a shutdown function, a lithium cobalt oxide-containing positive electrode, and a negative electrode containing graphite is analyzed in the nail penetration test, the separator shuts down ionic current flow in the short circuited region by the collapse of micropores caused by the Joule heat generated when it is heated to a temperature of approximately 135° C. However, there was still a problem of continued rise in battery temperature until the shutdown function is executed.
If the battery surface temperature rises continually, the temperature of the electronic devices using the battery rises. In such a case, the heat may affect the reliability of the electronic devices. Accordingly, it is desired to prevent heating of the battery by short circuiting, specifically to control the battery's maximum reachable temperature, for example, to about 80° C. or lower.
It was not possible to prevent the rise in battery surface temperature sufficiently in the nail penetration test, even when the method disclosed in Patent Document 1 of controlling the short-circuit current and reducing the Joule heat generation during internal short-circuit by using a low-conductivity (higher resistance) cathode material. For example, Patent Document 1 describes that LiCoO2, which has a higher resistance when the powder resistances of LiCoO2 and LiNiO2 are compared in the discharged state comparison, is more resistant to internal short circuiting (paragraph [0012]). Thus in Patent Document 1, only the conductivity of the cathode material in the discharged state was studied. However, when charged to some extent, LiCoO2 has a significantly different powder resistance, which is similar to or lower than the powder resistance of LiNiO2 in the charged state. Therefore, it is difficult to control the short-circuit current in a battery in the charged state sufficiently, only by specifying the powder resistance of the cathode material.    Patent Document 1: Japanese Patent No. 3362025